The cells forming porous materials include open cells and closed cells. The closed cells, the latter, are completely held out of communication with the outside, while the former, i.e., the open cells, are predominantly through voids permitting the passage of fluids therethrough and also include open voids having an inlet but no outlet. The ratio of the volume of open cells of the porous material to the volume of its mass will be referred to as an open cell ratio, which can be determined by experiments.
Processes for producing porous materials having such open cells (hereinafter referred to as "open-cell porous materials") include, first, a process comprising sintering a green molding of ceramic and/or metallic particles or fibers while producing open cells at the interstices between the particles or fibers, second, a process for vitrifying a green molding of ceramic and/or metallic particles and a vitrifying binder admixed with the particulate material while producing open cells at the interstices between the particles, third, a process for sintering a green molding of ceramic particles and a large quantity of combustible organic material admixed therewith while burning the organic material to form open cells, fourth, a process comprising heat-treating a melt of glass composed of a component insoluble in a chemical and a component soluble therein at a phase separation temperature and further dissolving out the soluble component with the chemical for removal to form open cells, etc.
These open-cell porous materials must have satisfactory gas or liquid permeability, which in turn requires a high open cell ratio. Further these open-cell porous materials are used under versatile conditions which require high strength. However, the conventional production techniques have encountered difficulties in giving both a high open cell ratio and high strength to open-cell porous materials since these characteristics are in conflict with each other.
Generally in producing open-cell porous materials, the open cell ratio increases but the strength decreases when the sintering temperature or vitrifying temperature is lowered, whereas the strength improves but the open cell ratio decreases when the sintering temperature or vitrifying temperature is conversely raised. On the other hand, the open cell ratio increases but the strength decreases when the molding density of green products is lowered, whereas a higher molding density results in improved strength but a decreased open cell ratio. Further when the proportion of the combustible organic material or of the soluble component of glass serving as a cell forming agent is increased, the open cell ratio increases but the strength lowers, whereas a decreased proportion of the cell forming agent entails improved strength but a decreased open cell ratio.
Thus, in the conventional techniques for producing open-cell porous materials, the increase in the open cell ratio and the improvement in the strength are conflicting characteristics. An object of the present invention, which has been accomplished in view of this problem, is to provide a process for giving an open-cell porous material an increased open cell ratio and improved strength.